The present invention relates to a process for filling a liquid crystal electro-optical device with a liquid crystal material or a mixture of a liquid crystal material and an uncured resin. The present invention also relates to a process for filling a liquid crystal electro-optical device with a liquid crystal material or a mixture of a liquid crystal material and an uncured resin, while avoiding the volatilization of low-molecular weight components such as low-molecular weight liquid crystals and uncured resins.
In general, a liquid crystal electro-optical device comprises, as the constitution thereof, a liquid crystal cell as follows.
A liquid crystal cell comprises a first substrate obtained by applying an alignment film to a substrate having thereon an electrode, baking, and subjecting the resulting substrate to an alignment treatment (referred to hereinafter as "rubbing"). A second substrate is fabricated similarly by applying an alignment film to a substrate having thereon an electrode, baking, and subjecting the resulting substrate to rubbing. The first and the second substrates thus fabricated are provided in such a manner that the electrodes thereof may be disposed opposed to each other, and a liquid crystal material is interposed between the substrates.
Depending on the liquid crystal that is used in the cell, there are cases in which an alignment film is not provided to the second substrate. In such cases, the liquid crystal material is provided directly in contact with the substrate or the electrode.
FIG. 1 shows a state of the liquid crystal cell before injecting a liquid crystal material.
Referring to FIG. 1, a liquid crystal cell comprises light-transmitting substrates 111 and 110 having thereon electrodes 112 and 113, and at least one of the substrates comprises formed thereon an aligning means 114 or 115 to align the liquid crystal in one direction. The spacing between the substrates is controlled to be uniform by means of a silica spacer 118. The substrates are fixed using a sealing material 119.
In general, a liquid crystal material is incorporated between the substrates of the liquid crystal cell by means of vacuum filling.
Vacuum filling method comprises fabricating a vacant cell with a filling hole provided in the sealing material to fill the cell with a liquid crystal material therethrough, evacuating the inside of the cell, and bringing a liquid crystal material into contact with the filling hole while maintaining the outer atmosphere at a positive pressure. The vacuum filling method is widely utilized in practice, because it is simple, and is suitable for mass production.
FIG. 2 shows the state of the liquid crystal cell after injecting a liquid crystal material.
Referring to FIG. 2, it can be seen that the molecules of the liquid crystal material 116 interposed between the substrates of the liquid crystal cell shown in FIG. 1 are aligned along the aligning means 114 and 115.
In case of vacuum filling, the vacant cell and the liquid crystal material are placed under vacuum to realize a vacuum state inside the cell to remove air from the liquid crystal material (this process is referred to hereinafter as "degassing").
More specifically, a sufficiently degassed liquid crystal material is brought into contact with the filling hole of the cell whose inside is evacuated, and the degree of vacuum is slightly lowered to increase the external atmosphere to a positive pressure. In this manner, the inside of the cell can be filled with the liquid crystal material.
A degree of vacuum necessary to obtain a vacuum cell and to degas the air inside the liquid crystal material is in a range of from 10.sup.-1 to 10.sup.-3 Torr, though depending on the size of the cell.
Widely known liquid crystal electro-optical devices include those of twisted nematic (referred to hereinafter as "TN") type or those of the super twisted nematic (referred to hereinafter as "STN") using nematic liquid crystals as the liquid crystal material. The nematic liquid crystals have high fluidity, and can be readily charged into the cell by vacuum filling at room temperature.
With increasing refresh rate and area of displays, more attention is paid to ferroelectric liquid crystals and antiferroelectric liquid crystals which respond at a rate three times as high as that of nematic liquid crystals.
The ferroelectric and the antiferroelectric liquid crystals exhibit smectic phases at room temperature, and have little fluidity. Thus, they must be fluidized so that they may be injected into the cell. Accordingly, they are heated until they exhibit a nematic phase or an isotropic phase.
Furthermore, the use of liquid crystal electro-optical devices equipped with dispersion-type liquid crystals which utilize light at higher efficiency and which are free of polarizer sheets is increasing. A liquid crystal electro-optical device of this type can be operated free of alignment films and without applying rubbing. They can be obtained by simply adhering substrates each having thereon an electrode, and interposing therebetween a resin binder with a plurality of liquid crystal particles several micrometers in diameter being dispersed therein. Although depending on the type of the liquid crystal, the liquid crystal is injected by a method similar to that used for other liquid crystals.
According to the invention of the present inventors disclosed previously, i.e., Japanese patent application No. Hei-6-45255, a method for increasing the physical strength of a liquid crystal electro-optical device is proposed, said method comprising mixing a liquid crystal material with an uncured resin material and filling the cell with the resulting mixture, wherein, the resin is separation precipitated from the liquid crystal into column-like cured products while aligning the liquid crystal material at the same time, thereby adhering the substrates to each other.
Another invention which comprises forming similarly a cured resin between the liquid crystal material and the substrate and alignment film is disclosed in Japanese patent application Nos. Hei-5-205887, Hei-5-209060, and Hei-6-80939. This invention enables ferroelectric and antiferroelectric liquid crystals to display a continuously changing intermediate gradation between a bright state and a dark one similar to that obtained on a nematic liquid crystal, and yet, without forming any domains.
The cell can be filled with a liquid crystal material described above or a mixture of a liquid crystal material and an uncured resin material (simply referred to hereinafter as a liquid crystal mixture) at room temperature if the liquid crystal material or the liquid crystal mixture exhibits excellent fluidity at room temperature. If not, the liquid crystal material or the liquid crystal mixture is heated to render fluidity thereto.
A liquid crystal material is a mixture of a plurality of liquid crystal materials, and it may contain a low molecular component having a molecular weight of about 1,000 or less. The lower molecular components tend to volatilize under vacuum, and this occurs more frequently with elevating temperature of the material.
The monomer molecules constituting the uncured resin material, however, are low molecular compounds having a very low molecular weight generally in a range of about 1,000 or less, more specifically, about 100 to 200. Accordingly, they tend to volatilize in the same manner as the low molecular components of the liquid crystal material.
However, in the vacuum filling method, the liquid crystal material or the liquid crystal mixture must be exposed to a vacuum at a degree of about 10.sup.-1 Torr in the filling step thereof. As a matter of course, the volatilization of the low molecular components was found unavoidable in this step.
The volatilization of low molecular components changes the properties of the liquid crystal material. Accordingly, this sometimes resulted in a failure in obtaining a liquid crystal material having the desired characteristics.
Furthermore, in case the concentration of the low molecular components of the uncured resin materials in the liquid crystal mixture decreases due to the volatilization thereof, the characteristics attributed to the presence of resin are sometimes found to be lost. Particularly, the intermediate gradation was found unobtainable in the second invention described above.
Thus, in the light of the aforementioned circumstances, the present invention aims to provide a process for fabricating a liquid crystal electro-optical device, which comprises injecting a liquid crystal material or a mixture thereof with a resin material by bringing it under vacuum or by heating under vacuum. The process realizes liquid crystal electro-optical devices having the desired characteristics with high reproducibility, and yet, prevents the volatilization of low molecular components of the liquid crystal material or the resin material from occurring.